The invention relates to a new process for the solvent-free production of hydroxy-functional copolymers, their use as a binder component in polyurethane powder coatings, the powder coatings produced using these copolymers and the use of these powder coatings to coat heat-resistant substrates.
Hydroxy-functional copolymers (polyacrylates) have in recent years won a secure position in the coatings sector. For ecological reasons, the reduction of the organic solvents necessarily used in the production and processing of hydroxy-functional polyacrylates is becoming increasingly important.
Hydroxy-functional copolymers are usually produced in inert organic solvents. In the course of the so-called continuous addition process, the bulk of the solvent is charged into a polymerization reactor and heated to the desired polymerization temperature. Subsequently, the monomer mixture and the initiator are charged simultaneously into the reactor at constant temperature and the desired copolymer is formed in the charged solvent by radical-initiated copolymerization. After charging of the monomer mixture and of the initiator is complete, stirring is continued for a certain additional period in order to complete the monomer conversion. The polymer solutions obtained usually have a narrow molecular weight distribution and can be used immediately after their production for the application concerned.
If, however, it is desired to recover and process solvent-free polymers, it is necessary to remove the solvent. The solvent is usually removed by distillation or in evaporation units. This procedure requires a considerable expenditure on equipment. Also, the recovered solvent cannot be used for further polymerization reactions without reprocessing, because it is usually contaminated with residual monomers.
One process for the production of solvent-free, hydroxy-functional polyacrylates is bulk polymerization in continuous reactors. In this process, the high equipment cost is disadvantageous, because the monomers which were not converted in the polymerization process must be distilled off, quantitatively determined and recharged to the polymerization process in the correct proportion. Continuous bulk polymerization processes are described, for example, in EP-A-00 68 024, EP-A-00 96 901, EP-A-01 56 170, U.S. Pat. No. 4,414,370, U.S. Pat. No. 4,529,787 and U.S. Pat. No. 4,546,160.
Such bulk polymerizations can, however, also be carried out discontinuously with evaporative cooling. Such processes are described, for example, in DE-OS 1 965 740, DE-AS 1 771 374 and DE-AS 2 422 043. The disadvantages of these processes are the restriction of the polymerization temperature and the fluctuation of the polymerization temperature during the polymerization. The highly exothermic reaction presents considerable cooling problems because the dissipation of the heat of reaction is made difficult by the large increase of viscosity.
A further possibility for the production of solvent-free, hydroxy-functional polyacrylates is the use of "reactive solvents", which first serve as reaction medium for the radical polymerization of the monomers and subsequently are added by a polymer-like reaction to the polyacrylate formed (PCT Application, Publication Number: WO 90/03991). In this case, .epsilon.-caprolactone serves as the "reactive solvent". After the radical polymerization, the .epsilon.-caprolactone is added by catalytic ring-opening polymerization to the OH groups of the polyacrylate with formation of new hydroxyl groups. The disadvantages of this process are (1) two consecutive reaction stages are necessary in order to obtain the desired solvent-free final product, and (2) for the second reaction stage, zinc catalysts are necessary. These catalysts are still present in the final product and during combination with, e.g., polyisocyanates, catalyze the urethane reaction and significantly reduce the pot life of the lacquer preparations concerned. As a result of this polymer-like reaction, the molecular weight and the viscosity of the polymers increase and disadvantageously affect the stirrability of the product in the reaction vessel.
Another kind of polymerization reaction in "reactive solvents" is described in EP-A-00 56 971. The reaction medium is a dialkyl maleate, which does not usually tend to homopolymerize. In this process, the low-viscosity dialkyl maleate is gradually incorporated into the polymer chain by reaction with the other monomers charged, so that at the end of the polymerization a nearly 100% product is present. Despite the good stirrability at the start of the polymerization, the formation of the high-molecular weight, almost 100% polymer leads to stirring problems towards the end of the reaction due to high melt viscosities.
The use of saturated or unsaturated polyesters as a reaction medium or grafting backbone provides another option for the production of solvent-free copolymers. Such processes are described, for example, in the patent applications EP-A-02 06 072 and EP-A-03 91 271. In this process, the high viscosity of the "reaction medium polyester" at the start of the process leads to poor dissipation of the heat of reaction and results in products with a very non-uniform molecular-weight distribution.